This invention relates a laser diode protecting circuit in a laser diode drive having an automatic current control circuit (ACC circuit) for performing control in such a manner that laser diode current attains a set value, as well as to a laser driving current control circuit in the above-mentioned ACC circuit. More particularly, the invention relates to a laser diode protecting circuit for protecting a laser diode by preventing an excessive emission from the laser diode when the laser diode is started up at low temperatures, as well as to a laser driving current control circuit applicable also to laser diodes of both the common-anode and common-cathode types.
A deterioration in transmission characteristics due to wavelength fluctuation (chirping) cannot be ignored in high-speed optical communications. In addition, wavelength stability is extremely important in wavelength division multiplexing. For these reasons the laser diode drive is constructed by combining an ACC circuit and an ATC (Automatic Temperature Control) circuit and control is performed in such a manner that the laser diode current will attain a constant current value and the laser diode chip temperature (laser diode temperature) a constant temperature.
FIGS. 24A, 24B are block diagrams illustrating optical transmitters used in digital optical communication, in which FIG. 24A shows an optical transmitter using a laser diode of the common-anode type, and FIG. 24B shows an optical transmitter using a laser diode of the common-cathode type. Numeral 1 in these Figures denotes a laser diode drive, 1a a common-anode laser diode and 1b a common-cathode laser diode. Also shown are an ACC circuit 2, which is constituted by an operational amplifier (OP amp) for performing control in such a manner that the laser diode current attains a set current value, an ATC circuit 3 for performing control in such a manner that the laser diode temperature attains a set value, optical fibers 4, 5, a D-type flip-flop (D-FF) 6 for storing a data signal DATA in response to a clock CLK, and a drive circuit (DRV) 7 for a light intensity modulator (IM) 8, which modulates light intensity in accordance with the "1", "0" logic of the data. The laser diodes are of common-anode type and common-cathode type, the driving currents of which have different directions. The laser diode 1a of common-anode type (FIG. 24A) has its anode connected to ground, and it is required that a driving current id be expelled from the laser diode 1a. The laser diode 1b of common-cathode type (FIG. 24B) has its cathode connected to ground, and it is required that a driving current id be drawn in by the laser diode 1b.
FIGS. 25A, 25B show examples of the ACC circuit 2, in which FIG. 25A shows an ACC circuit of common-anode type, and FIG. 25B shows an ACC circuit of common-cathode type.
In FIG. 25A, the laser diode (LD) of common-anode type is indicated at 1a. The ACC circuit includes resistors R1-R3 having resistance values r.sub.1 -r.sub.3, respectively, a transistor TR1 and a comparator (current control circuit) IC1 constituted by an operational amplifier. The laser diode 1a, transistor TR1 and resistor R1 are serially connected and provided between ground and a negative power source -Vee. If id represents a current that flows through the laser diode 1a, then id.cndot.r.sub.1 will enter the inverting input terminal of the comparator IC1. On the other hand, a reference voltage V.sub.REF, obtained by voltage division by the resistors R2, R3, enters the non-inverting input terminal of the comparator IC1. The ACC circuit 2 brings the laser diode current id into line with the set current value by controlling the on/off operation of the transistor TR1 in such a manner that the terminal voltage id.cndot.r.sub.1 across the resistor R1 becomes equal to the reference voltage V.sub.REF. More specifically, the voltage V.sub.REF obtained by voltage division by the resistors R2, R3 becomes the voltage across the resistor R1 and a value obtained by dividing this voltage by the resistance value r.sub.1 becomes the current id that flows through the laser diode 1a. In other words, the base of the transistor TR1 is controlled by the comparator IC1 in such a manner that the resistor R1 will serve as a constant-current source the current value of which will be V.sub.REF /r.sub.1 at all times, thereby making it possible to obtain a constant current value even when the temperature varies.
In FIG. 25B, the laser diode (LD) of common-cathode type is indicated at 1b. The ACC circuit includes resistors R4-R6 having resistance values r.sub.4 -r.sub.6, respectively, a transistor TR2 and a comparator (current control circuit) IC2 constituted by an operational amplifier. The laser diode 1b, transistor TR2 and resistor R4 are serially connected and provided between ground and a positive power source +Vcc. If id represents a current that flows through the laser diode 1b, then id.cndot.r.sub.4 will enter the inverting input terminal of the comparator IC2. On the other hand, a reference voltage V.sub.REF, obtained by voltage division by the resistors R5, R6, enters the non-inverting input terminal of the comparator IC2. This ACC circuit brings the laser diode current id into line with the set current value by controlling the on/off operation of the transistor TR2 in such a manner that the terminal voltage id.cndot.r.sub.4 across the resistor R4 becomes equal to the reference voltage V.sub.REF. More specifically, the voltage V.sub.REF obtained by voltage division by the resistors R5, R6 becomes the voltage across the resistor R4 and a value obtained by dividing this voltage by the resistance value r4 becomes the current id that flows through the laser diode 1b. In other words, the base of the transistor TR2 is controlled by the comparator IC2 in such a manner that the resistor R4 will serve as a constant-current source the current value of which will be V.sub.REF /r.sub.4 at all times, thereby making it possible to obtain a constant current value even when the temperature varies.
FIG. 26 illustrates an example of the ATC circuit. The laser diode chip is shown at 1a. The ATC circuit includes a Peltier device 3a for heating or cooling the laser diode chip 1a depending upon the direction of the current, and a thermister 3b having a negative resistance characteristic for detecting the temperature of the laser diode chip 1a. The laser diode 1a, Peltier device 3a and thermister 3b are accommodated in a package 3c. The ATC circuit further includes resistors 3d, 3e, PNP, NPN transistors 3f, 3g and a comparator 3h. A voltage Vt (which conforms to the laser diode temperature) resulting from voltage division by the thermister 3b and resistor 3d is applied to the inverting input terminal of a comparator 3h, and a reference voltage V.sub.REF is applied to the non-inverting input terminal of the comparator 3h. The output terminal of the comparator is connected to the bases of transistors 3f, 3g. The emitter of the PNP transistor 3f is connected to V+, the emitter of the NPN transistor 3g is connected to V-, and the collectors of these transistors are connected to the Peltier device 3a.
When the laser diode chip is at a low temperature, the resistance of the thermister 3b increases, the voltage Vt decreases to establish the inequality Vt&lt;Vref and the output of the comparator 3h becomes positive. As a result, the transistor 3f is turned off and the transistor 3g is turned on so that a current flows in a direction that causes the heating of the Peltier device 3a, thereby heating the interior of the package 3c and raising the temperature of the laser diode. When the temperature of the laser diode chip rises, the resistance of the thermister 3b decreases and the voltage Vt increases to establish the inequality Vt&gt;Vref so that the output of the comparator 3g becomes negative. As a result, the transistor 3f is turned on and the transistor 3g is turned off so that a current flows in a direction that cools the Peltier device 3a, thereby lowering the temperature of the laser diode. The temperature of the laser diode is thus controlled so as to attain the set temperature.
When power is introduced to the optical transmitters of FIGS. 24A and 24B at low temperatures to drive the laser diodes 1a, 1b, the laser diode emits radiation excessively and the laser diode itself may be damaged. The reason for the excessive emission is as follows: The laser diode has a temperature characteristic of the kind shown in FIG. 27. It will be understood that the lower the temperature, the greater the power P needed to pass a constant laser diode current. If ACC stabilization time at which the laser diode current attains the set value by ACC is compared with stabilization at which the laser diode temperature attains the set value by ATC, it will be seen that ATC stabilization time is longer than ACC stabilization time. Consequently, when the laser diode is driven by introducing power at low temperature, as shown in FIG. 28 the laser diode current attains the set value by ACC before the laser diode chip attains the fixed temperature owing to the delay involved in ATC, as a result of which the power of the emission from the laser diode increases and becomes so excessive as to degrade the characteristic of the laser diode and eventually destroy the same. In other words, though the laser diode current attains the target value owing to the ACC circuit, the laser diode temperature does not attain its target value. Accordingly, the laser diode produces an emission in excess of the target value. It is necessary to prevent the excessive emission from the laser diode at low driving temperatures so that the laser diode will not be destroyed or suffer degradation of its characteristics.
Further, the laser diodes are of the common-anode and common-cathode types, as mentioned above, the comparators (current control circuits) IC1, IC2 used in the respective ACC circuits (see FIGS. 25A, 25B) are different and they must be designed and provided separately. It would be advantageous, therefore, if the ACC circuits of each type could make common use of a current control circuit, and a reduction in cost can be achieved by making common use of the current control circuits (i.e., by using LSI techniques).
The minimum value of laser current id controlled by the ACC circuit is 0 mA. This specification stipulating a minimum value of 0 mA is necessary for implementing a shut-down function, namely a function for halting completely the emission of laser light necessary for an optical transmitter. Consequently, it is required that the ACC circuits of both types perform control in such a manner that the voltage produced across the resistors R1, R4 is made 0 V, resulting in that it is required that the range of input voltages of the operational amplifier of the shared comparator (current control circuit) include the positive and negative power-source voltage values (+Vcc, -Vee). In other words, if v represents the terminal voltage of the resistors R1, R4 produced by the laser current id at the time of an ordinary emission, it is required that the operational amplifier of the shared comparator (current control circuit) operates at least at an input voltage within the voltage range of +Vcc to (+Vcc-v) or -Vee to (-Vee+v) shown at (a) of FIG. 29. This input voltage range can be relaxed to some extent by enlarging the resistance values r.sub.1, r.sub.4 of the resistors R1, R4, respectively. However, when such factors as a reduction in the voltage of the circuit power source and the maximum value of controllable current are taken into consideration, it is desired that the resistance values r.sub.1, r.sub.4 be several ohms to several tens of ohms. Hence, there is a limitation on how large r.sub.1, r.sub.4 can be made.
The input voltage range of a typical operational amplifier is -Vee to (+Vcc-1.5) or (-Vee+1.5) to +Vcc, as shown at (b) or (c) of FIG. 29. The operational amplifier cannot operate when a signal within a range of about 1.5 V from the +Vcc or -Vee of power supply level is input to the amplifier. This is a characteristic that inevitably accompanies an operational amplifier constituted by a differential pair. That is, with a typical operational amplifier, it is difficult to obtain an input voltage range [the voltage range shown at (a) of FIG. 29] in the vicinity of both power source voltages required for the operational amplifier of the shared comparator (current control circuit).